If you make a duplicate of a frame (sprite) and edit it somehow, the original frame of which you made a duplicate gets altered as well and vice versa. Why doesnt it make a new independent copy that you could freely edit without changing the original frame?
Frames on a sprite object animation are just references to the files. Duplication just duplicates the reference.
If you want a new unique file. When you duplicate you will need to open the new one in piskel then use save as to select a new filename.
I know the principle behind it but it would be more convenient if duplication created a unique file.